The present invention pertains to strain gauge load cells, and, more particularly to single-ended or double-ended shear beam load cells having welded structural components which increase resistance to overload or side load forces.
Load cell weighing systems, and, in particular, weighing systems for vehicles such as logging trucks and bulk haulers, typically use a plurality of load cell assemblies to monitor the weight added to the trucks and trailers. Load cell assemblies usually include a machined steel block that is supported between load-carrying members, such as log supports, a bulk container or a tank, and mounting members, such as a truck or trailer frame. These steel blocks ar configured to direct the load to predetermined areas, where the load induced stresses are measured using strain gauges. These load cells are divided into two types, those which measure bending beam stresses and those which measure the shear stresses in the beam.
Bending beam load cells known in the art have inherent problems in safety and operation as detailed in U.S. Pat. No. 4,666,003 issued to Reichow, which is incorporated herein by reference. An additional problem is the non-linearity which occurs from even slight misalignment of the strain gauges for the bending beam load cells.
Weighing systems using the shear beam load cells have typically been manufactured by machining a hole partially through the side of a solid beam in transverse directions, leaving a web in the middle of the beam. The strain gauges are then placed on the center of the web. The resulting structure resembles an I-beam in cross-section, and resists forces directed downwardly. However, this load cell configuration is weak when side forces are applied, especially off-center side forces. The resulting twisting of the load cell results in reduced safe side load operation.
All of the prior art load cells avoid the use of structural welding in areas through which forces are directed for strain gauge measurement. U.S. Pat. No. 4,020,911 issued to English, et al. shows structural welding, but only in the heavy beam areas which serve as rigid mounting for the load cell.
Delicate strain gauges and wiring require protection from damage which occur when service technicians clean load cells by scraping mud and dirt and chipping ice therefrom. Welded plates would be an attractive solution to this type of strain gauge damage but for the fact that the prior art teaches away from welding cover plates in the area of the load cell where the strain gauges are located. Thus, the prior art and techniques include the use of chemical sealing compounds which are easily breached.
Where cover plates have been employed in the prior art, they have been non-structural supporting elements. U.S. Pat. Nos. 4,838,372 and 4,858,710 use welding to fasten and seal nonsupporting cover plates. However, the load is carried by the machined load cell block. These two patents limit the type of welding which can be used to precision welds such as microplasma welding, laser welding or electron beam welding, Thus, the welding is meant to seal and fasten the parts together, but is not intended to carry more than a very small portion of the applied load. More specifically, the configuration of these two prior art load cells has been dictated by the necessity of avoiding the welding of cover plates in an area of the load cell which bears the structural load. Specifically, U.S. Pat. No. 4,838,372 and U.S. Pat. No. 4,858,710 both disclose load cells which have an aperture 2 (i.e. a through hole) with welded supports 3 and 6 (i.e. cover plates) thereon. The aperture 2 (through hole) is not parallel with the direction of applied force, but is instead oriented perpendicular thereto to avoid loading of this area. Also, welded supports 3 and 6 are oriented in planes parallel with, not perpendicular to, the direction of the applied force so that these welds and supports do not bear the structural load.
U.S. Pat. No. 3,602,866 teaches a load cell which measures bending tensile stresses in the top and bottom of the beam, as opposed to shear beam load cells. A force transducer has a beam 10 with a hole 16 (i.e. a through hole) formed therein. Strain gauges 22 and 24 are bonded to the opposite sides of hole 16 . Holding plate 37 is fixedly secured over the hole 16 to enclose the strain gauge elements 22 and 24 without interfering with the flexural qualities of the beam 10. In order to avoid loading of the holding plate, the through hole is oriented perpendicular to, not parallel with, the applied force; and the holding plate is oriented in a plane parallel with, not perpendicular to, the direction of applied force.
A need thus exists for a load cell in which the welding, or other means for securing the chamber cover plate to the load cell, carries the structural loads of the load cell; thus providing increased resistance to overload and side load forces, and to bending and twisting of the load cell.
A need exists for the above type of load cell in which the welds are in an area of low shear stress so that the effect of the welding on the measured strain is minimized.
A need exists for the above type of load cell where the chamber cover plate also carries the structural load of the load cell.
A need exists for the type of load cell herein described in which the chamber cover plate is structurally welded onto the load cell in an orientation whereby the chamber cover plate extends transversely to the direction of the applied force and to the axis of the chamber so that the chamber cover plate and structural welds carry the structural loads of the load cell.